You are doing the validation on client click OnClientClick="ValidComment()" . In the "ValidComment" you are just checking and showing the alert box. but the OnClientClick is expecting the boolean value "TRUE" or "False". You have to return "True/False" in the validation function. So that its not working.

If you are not returning anything by default it will take as "True". So you need to send "False" value based on the validation.By NathanDirection is important than speed